1. Field of the Invention
The present invention relates to an ion implantation method, and more particularly, to an ion implantation method for achieving angular uniformity throughout a workpiece being implanted, and application thereof.
2. Background of the Related Art
Ion implantation is a material engineering process by which ions of a material can be implanted into another solid, thereby changing the physical properties of the solid. Ion implantation is used in semiconductor device fabrication. Referring to FIG. 1, which is a schematic illustration of an ion implantation system 100 in accordance with the conventional art. The ion implantation system 100 comprises an ion beam source 110, a workpiece 120 to be implanted, and a workpiece holder (not shown). An ion beam 130 is generated and directed by the ion beam source 110 to the surface of the workpiece 120. The workpiece holder may be designed that during the implantation the workpiece 120 is allowed for both a linear motion and a rotational motion, as indicated by arrows 140 and 150.
Referring to FIG. 2, which is a schematic illustration of ion implantation operation in accordance with the conventional art. An ion beam 210 is directed to strike the surface of a workpiece 220. The ion beam 210 may comprise at least one beamlet 230 (including a center beamlet 231). A beam spot 211 is formed on the workpiece surface 220 by individual beamlets 230 hitting on the workpiece surface 220. Each beamlet 230 may strike the workpiece surface 220 with a corresponding incident angle 240. The incident angle 240 is an angle between the incoming beamlet 230 and the normal (indicated by the dotted line 221) to the workpiece surface 220. It is noted that in this example the center beamlet 231 is in parallel with the normal 221 to the workpiece surface 220, which is not necessary. It is also noted that the beamlet 230 is depicted a straight line merely for convenience of illustration, in reality they are mostly curved lines except for the center beamlet 231.
In ion implantation, it is important to control the beamlet incident angle 240 because the implantation depth is generally a function of the incident angle 240 as a result of the well-known channeling effect. Further, it is sometimes necessary to perform the implantation at certain incident angles 240 in order to conform to the geometry of the device being manufactured.
In semiconductor device fabrication, the wafer dose uniformity and angular uniformity are critical to the electrical characteristics of the products. To achieve the dose uniformity throughout a workpiece being implanted, numerous implantation methods have been proposed, such as the continuous rotation-implantation approach which alternately performs rotation and implantation of the target workpiece such that when the implantation process is complete, a dose uniformity throughout the workpiece is achieved.
While it is possible to design beam transport optics which are nearly aberration-free, the beam characteristics such as angular divergence is nonetheless still determined to a great extent by the emittance properties of the ion source. A divergent beam is oftentimes fetal to the angular uniformity of the implanted workpiece in ion implantation. Therefore, an implantation method capable of achieving the angular uniformity with the limited ability of current system hardware is desired.